The principal function of automotive steering wheels is, of course, steering the car. In addition, steering wheels which are kept in contact with the driver's hands while the car is running must satisfy several other requirements, among which the following are important: (1) good appearance, (2) good touch to the hands of the drivers, (3) high resistance to ultraviolet radiation, and (4) high resistance to wear due to constant friction with the hands of the drivers.
"Soft touch" steering wheels (that is, steering wheels which are soft to the touch) that satisfy these requirements are inclined to be desired by drivers in recent years, and as a consequence, the use of "rigid" resins such as polypropylene and butyl cellulose as materials of steering wheels is shifting to the adoption of foamed polyurethane.
The polyurethane foams used in the prior art are formed from expensive raw materials such as polyols and isocyanates. In addition, they are shaped into steering wheels by complex forming methods. For these reasons, steering wheels made of polyurethane foams are costly. A further problem with polyurethane foams is that the molecular chains of polyurethane are easily broken upon exposure to ultraviolet radiation, and therefore, the skin of a steering wheel made of such polyurethane foams has a tendency to deteriorate as a result of prolonged use.
Soft (flexible) vinyl chloride resins are currently used as the material for steering wheels having a soft touch and being inexpensive. However, non-foamed vinyl chloride resins have a comparatively high hardness and do not fully meet the demand of users requiring the soft touch steering wheels. If a steering wheel is formed by foamed vinyl chloride resins using an efficient injection molding method, defects such as "flow marks" or "swirl marks" are caused in the molded product. These defects extend to such a deep area of the product that they cannot be completely masked by any subsequently applied coating. Forming techniques other than the injection molding methods are inefficient and costly. Therefore, no manufacturer has yet succeeded in making steering wheels from soft (flexible) vinyl chloride resins having a hardness of 60 Hs or less that can be produced at low cost (cost effectiveness is one of the important factors in mass production of automotive parts) and which have soft touch that is comparable to that of products made from polyurethane foams.